The invention relates generally to an interferometric method for testing large convex (spherical or aspheric) mirror surfaces. These convex mirrors are typically used as the secondary mirrors of Cassegrain and related telescope types such as Ritchey-Chretien. The invention is a further improvement on the methods described in U.S. patent application Ser. No. 12/343,683 filed Dec. 24, 2008 and Ser. No. 12/467,278 filed May 17, 2009, wherein the mirror is initially treated as a lens to obtain a precise surface figure for the convex surface prior to applying a reflective coating. These applications are hereby incorporated by reference. While this invention is primarily concerned with testing large (or small) convex mirror surfaces, the same type of test can be applied to testing lenses, in particular lenses with at least one convex surface (spherical or aspheric).
Large convex mirrors are typically used as secondary mirrors in large reflecting telescopes. For example, the NASA 3-meter telescope on Mona Kea uses a 244-mm diameter secondary mirror having a hyperbolic surface figure. Currently the standard method for testing convex mirrors is the Hindle sphere test or the improved version, the Hindle-Simpson test. The Hindle test uses a spherical mirror that is significantly larger in diameter than the convex mirror under test and it must be perforated at its center. A diagram of the test set-up is shown in FIG. 1.
The convex mirror under test, the test optic 10, is tested at the same conjugates as used in the telescope by employing a Hindle Sphere 11, a spherical mirror with a central perforation. The center of curvature (CoC) of the Hindle Sphere is positioned at the near focus 12 of the convex surface under test. The diameter of the Hindle Sphere has to be greater than that of the test optic. Light from an interferometer 13 is brought to the null test point 14 at the far focus of the convex surface of the test optic. After reflections off the test optic 10 and the Hindle Sphere 11, the light re-traces its path back to the interferometer 13 where it produces fringes on a monitor 15 depicting the wavefront aberrations of the test optic.
A schematic of the Hindle-Simpson test set-up is shown in FIG. 2. This test makes use of a meniscus-shaped Hindle Sphere 20 and a concave calibration mirror 21. All surfaces in the arrangement are spherical. By designing the ancillary optics, in this case the meniscus-shaped Hindle Sphere and the concave calibration mirror, to lie close to the convex mirror under test 22, the diameters of these optics are minimized with a corresponding reduction in the cost of fabrication. Nonetheless, the diameters still have to be somewhat larger than the diameter of the test optic.
In large telescopes, astronomical or otherwise, the secondary mirror often directs the light to a focus through a central hole in the primary. The distance from the vertex of the secondary mirror to this focus can be many meters, perhaps more than 10 meters. To reduce the total length of the test setup, a shortening lens 30 is often used as shown in FIG. 3. The lens is often a plano-convex lens with spherical convex surface. Again, this lens has to have a diameter greater than the diameter of the mirror under test 31, further adding to the complexity and cost of the test setup.
The Hindle-Simpson test is not easily or cheaply implemented because of the large diameter requirements for the Hindle-Sphere—a meniscus element, and a calibration mirror—a concave spherical mirror. The diameters of both must be larger than the diameter of the test optic. For example, Hindle-Sphere and Calibration Mirror diameters for testing the 1.4-meter diameter convex hyperboloid secondary mirror for the F/15 Keck Telescope Secondary Mirror both have to be at least 1.4 meters. Each optical surface of these large ancillary components must all be precisely figured and polished.
Computer Generated Holograms (CGH) can be used for testing large convex mirror surfaces. Again, the CGH diameter has to be larger than the test optic diameter. Therefore, this approach does not eliminate the requirement for large expensive ancillary optics.
The previously mentioned test methods for the surface figure of large convex mirrors (U.S. patent application Ser. Nos. 12/343,683 and 12/467,278) requires a substrate with good optical homogeneity. The front side is shaped and polished to approximately the desired convex curvature and the rear side must be shaped to a simple but precise surface figure, such as an optical flat. In the present invention, the substrate material need not be of good optical quality nor does the rear side have to be shaped to a precise surface figure. Both of these characteristics will be measured and taken into account when determining corrections to the front surface figure. A further benefit of the test method described here is that ancillary optical component diameters are significantly smaller that the test optic diameter, usually by a factor of about 10×. Assuming substrate weight proportional to test optic diameter raised to the third power, the weight of these ancillary optics is generally about 1000× less than that of equivalent Hindle-Sphere ancillary optics. The size and weight reductions in the ancillary optics afforded by the test method leads to optically precise interferometric tests setups that can be constructed and implemented rapidly at significantly reduced cost.